Greetings! First post here, looking for some help.
I've got a Trane XV80 that is not producing any burner flames. I've read a number of posts and watched a bunch of videos and this is where I've got to:

Sequence
1) Thermostat calls for heat.
2) Draft inducer starts running.
3) Pressure switch proves vacuum from draft inducer.
4) Igniter starts heating as long as pressure switch has proven.

And here things hang. I do not think the gas valve is opening.

Furnace makes three attempts to ignite, then goes into "2 flashes" EXTERNAL LOCKOUT (RETRIES OR RECYCLES EXCEEDED)

That seemed to indicate the gas valve or the control board. I checked voltages at several places on the control board and find 26-28 VAC. I checked voltage between C and Hi on the gas valve and get only a few mV. Same between C and M, with a brief flash to about 16 VAC. Seems like the control board.

By lucky circumstance I had another control board. I put that in and turned the furnace on. Start-up went smoothly, igniter came on, gas valve opened, and the burners lit!

The burners stayed on for about 30 seconds, then went out, and we are back to where I started. The start-up sequence proceeds through the igniter glowing, no burners ignite, the system resets, and the cycle repeats.

Now what? What else could be preventing the control board from opening the gas control valve? Is the gas control valve coil bad? Could it be damaging the control board? How can I test that?

 

It sounds to me like you have already proven your theory.
You need a gas valve and a board.

I would still isolate the wiring to the control valve (unplug it) , shut off the gas service valve, ( not at the control valve) and measure the resistance between Hi and Com and then Low to Com on the gas valve. I would then measure High to ground and Low to ground.

 

I turned the manual gas supply valve off. Thank you.

Hi to C measures 22 MOhms
M to C, and everything to ground measure off scale.

 

After the igniter glows there may be a quick voltage rise measured between Hi and C at the gas control valve input connector, with the connector disconnected from the valve. I can't get a good reading with a multimeter and don't have an oscilloscope.

 

If you can't get the momentary 24v on the gas valve then you need to check at the multiwire connector on the board. You shouldn't need a scope. You need hard 24vac. Some typical problems are those connectors get corroded. I spray a little contact cleaner into the plug only and then reinsert it several times. If you had voltage at the valve then I'd recommend operating the power switch a few times. You could also clean the plug there too.

 

Tried some contact cleaner spray. No change.

I wonder if the control board senses the load, and if the load isn't as expected it does not apply voltage....

 

Unplug the low voltage to the gas valve and measure for 24 volts after the ignitor energizes to check.

 

It turned out to be the gas control valve AND the control board.

And after replacing those, then the igniter burned out!

 

Thanks for the follow up on the diagnosis.